Add yamllint to the www-gitlab-com image

python3-pkg-resources was also added as a dependency, because there's bug in Debian Stretch http://debian.2.n7.nabble.com/Bug-917271-yamllint-requires-module-pkg-resources-but-does-not-depend-on-it-td4440193.html

Needed for gitlab-com/www-gitlab-com!47628 (merged)